1. Field of the Invention
The present invention relates to a magnetic recording medium that is mounted on various magnetic recording apparatuses including external recording devices for computers, and to a manufacturing method thereof.
2. Description of the Related Art
Magnetic recording media are desired to have increased recording density and reduced noise. Accordingly, various compositions and structures of a magnetic layer, materials for a nonmagnetic base layer, and the like have been proposed. In particular, in recent years, the magnetic layer commonly referred to as a “granular magnetic layer” has been proposed in which a nonmagnetic nonmetallic substance, such as an oxide or nitride surrounds a periphery of ferromagnetic crystal grains.
For example, Japanese Patent Application Laid Open Publication No. 8-255342 proposes that noise be reduced by sequentially stacking a nonmagnetic film, a ferromagnetic film, and a nonmagnetic film, and then heating the stack to form the granular recording layer in which ferromagnetic crystal grains are dispersed in the nonmagnetic film. In this case, the nonmagnetic film is composed of a silicon oxide, a silicon nitride, or the like. Further, U.S. Pat. No. 5,679,473 describes a method of using a CoNiPt target doped with an oxide such as SiO2 to carry out an RF sputtering in order to form a granular recording film in which magnetic crystal grains are separated from one another and surrounded by a nonmagnetic oxide, thereby achieving reduced noise.
A reason the granular magnetic film features reduced noise properties has been assumed to be that a nonmagnetic, nonmetallic grain-boundary phase physically separates the magnetic grains from one another to weaken a magnetic interaction among the magnetic grains, thereby hindering zigzag magnetic domain walls from being formed in recording-bit transition regions. A conventional CoCr-based metallic magnetic film is formed at high temperatures, so that Cr is segregated from Co-based magnetic grains and deposited at a grain boundary to weaken the magnetic interaction among the magnetic grains. However, in the granular magnetic layer, the grain-boundary phase is a nonmagnetic, nonmetallic substance that is more likely to be deposited as Cr in the prior art, thereby advantageously allowing the magnetic grains to be isolated relatively easily. In particular, for the conventional CoCr-based metallic magnetic films, it is essential to increase the temperature of the substrate to 200° C. or higher during film formation in order to sufficiently deposit Cr. In contrast, there is the advantage that in the granular magnetic layer, even if film is formed without heating, the nonmagnetic, nonmetallic substance produces segregation.
Further, to increase the recording density and reduce the noise of a magnetic recording medium, an inter-grain magnetic interaction should be weakened by facilitating segregation in the magnetic layer, and a crystal orientation of the CoCr-based ferromagnetic crystal grains should be controlled. Specifically, the c-axis of the hexagonal close-packed ferromagnetic crystal grains should be oriented in a film surface. For this purpose, in a case of the prior art metallic magnetic layers, the crystal orientation of the magnetic layer is realized by controlling a structure and crystal orientation of the nonmagnetic base layer.
On the other hand, an effect of the nonmagnetic base layer has been considered small in magnetic recording media having a granular magnetic film, because the nonmagnetic base layer is separated from the ferromagnetic crystal grains by boundary segregation materials such as oxide. Journal of Magnetic Society of Japan, Vol. 23, No. 4-2, p. 1021 (1999) discloses that if a specific-composition CrMo alloy in which a (110) plane is favorably oriented and is used in the base layer, then (100) and (101) planes of the ferromagnetic crystal grains in the granular magnetic layer are favorably oriented and the magnetic properties and electromagnetic conversion properties are improved.
However, if the (101) plane of the ferromagnetic crystal grains is favorably oriented, a c-axis is not parallel with the film surface but stands on the film surface at a certain angle. Then, a magnetic anisotropy of the crystal grains has an element normal to the film surface, and a resulting vertical element of magnetism increases the media noise. Such a favorable orientation of the (101) plane of ferromagnetic crystal grains is caused by the favorable orientation of the (110) plane of the CrMo-alloy base layer. Accordingly, a control of the crystal orientation of the magnetic layer disclosed in the Journal of Magnetic Society of Japan is not sufficient. Thus, there is a need to precisely control the crystal orientation further to realize a medium of low noise.
We have disclosed in Japanese Laid Open Patent Application No. 2000-376597 that the orientation of the base layer and magnetic layer can be controlled by forming an amorphous seed layer and an orientation control layer on the nonmagnetic substrate.
For further improvement of the magnetic properties and reduction of noise, the magnetic layer is required to have stronger in-plane orientation and to achieve high-quality epitaxial growth. Thus it is necessary to optimize materials for the nonmagnetic base layer and the nonmagnetic intermediate layer to minimize misfit and ease downsizing of the particle size.